4 ms·

I just had a look at the 'guts' of your project box, please make sure that you maintain minimal spacing between the high voltage and low voltage parts of your c

by 0x12 15y ago

I just had a look at the 'guts' of your project box, please make sure that you maintain minimal spacing between the high voltage and low voltage parts of your circuit. As it is it looks like they are much too close to each other. Just having them electrically separated is not enough, you have to account for arcing in all kinds of conditions as well as temporary over voltage (spikes) on the current carrying wires.

theatrus2 15y ago
[Disclaimer: I've put products successfully through UL] The OP is correct, for a non-isolated product (one with user exposed metal bits and connectors) you're not paying enough attention to separation and protection. UL would probably require double insulated wires, wire guides, and moving your wire lug away from logic circuits, adding a fuse, along with all sorts of requirements on labeling, etc etc. Also, are you calculating RMS current or average current? Without a voltage tap, you also can't determine power due to power factor.
- trafficlight 15y ago
  Do you have any recommended documentation for doing these things properly? I'm pretty naive when it comes to this stuff, so any information about best practices would be appreciated.
  - 0x12 15y ago
    I think the easiest way for you to significantly improve the circuit from a safety perspective is to modify the layout in such a way that the high voltage part of the circuit is on one end of it and the low voltage part on the other. A (very safe) alternative to using the hall sensor the way it is wired up right now: consider wrapping the current carrying wire around a ferrite core and putting a number of turns around the other side of the core to pick up the current (a very basic transformer). That way there is absolutely no need to go 'near' to the low voltage part of the circuit with the high voltage portion of the wiring. That's called a current sensing transformer. You could also place the hall sensor if you really want to use that in the gap of a slotted toroid around which the primary wire is wound a number of turns. Adding a fuse is a basic precaution, you should size it based on the wire thickness that you are using. You are doing pretty good but beware of knowing 'just enough to be dangerous'. Alternatively once you've finished prototyping the circuit you could hand off the process of packaging it in a way that complies with the various standards to an electrical engineering firm, from a liability perspective that might be a good idea anyway. hth
- jrockway 15y ago
  I don't think he cares about accuracy of the result; all he needs is a binary on off. So you measure the output of the circuit with the TV off, make a note, measure the output of the circuit with the TV on, make a note, and set the threshold (in software), somewhere in between. Then you know if the TV is off or not. (I would personally have pasted a photocell onto the power LED, if I really needed to know the TV's state, but that's just me. My TV doesn't have a button for "power on", but it responds to a remote code for one.)
lutorm 15y ago
I don't disagree with your advice, but in this case I think it's not so bad. It's a current sensor, there's practically no voltage across those power wires. And assuming he wired it into the ground part of the circuit and not the phase, no voltage to the electronics either. (However, low voltage doesn't mean that an accidentally dropped conductor wouldn't ruin your day, so insulating them would still be a good idea.)
- 0x12 15y ago
  If you look closely you can see a wire running from the mains straight to the sensor board next to the arduino, clearance is on the order of a few mm. All it takes is a little bit of condensation for a nasty accident, the voltage between those two conductors is 110V AC on one side and +5 V with some unknown level of insulation from mains on the other. Making assumptions about which line carries phase and which is neutral can cause very nasty surprises and should never be a factor. You're one broken circuit away from being the shortest path to ground, even in the neutral line (you'll still have the load as a drop resistor but it's not good).

by 0x12 15y ago

theatrus2 15y ago

[Disclaimer: I've put products successfully through UL] The OP is correct, for a non-isolated product (one with user exposed metal bits and connectors) you're not paying enough attention to separation and protection. UL would probably require double insulated wires, wire guides, and moving your wire lug away from logic circuits, adding a fuse, along with all sorts of requirements on labeling, etc etc. Also, are you calculating RMS current or average current? Without a voltage tap, you also can't determine power due to power factor.

trafficlight 15y ago
Do you have any recommended documentation for doing these things properly? I'm pretty naive when it comes to this stuff, so any information about best practices would be appreciated.
- 0x12 15y ago
  I think the easiest way for you to significantly improve the circuit from a safety perspective is to modify the layout in such a way that the high voltage part of the circuit is on one end of it and the low voltage part on the other. A (very safe) alternative to using the hall sensor the way it is wired up right now: consider wrapping the current carrying wire around a ferrite core and putting a number of turns around the other side of the core to pick up the current (a very basic transformer). That way there is absolutely no need to go 'near' to the low voltage part of the circuit with the high voltage portion of the wiring. That's called a current sensing transformer. You could also place the hall sensor if you really want to use that in the gap of a slotted toroid around which the primary wire is wound a number of turns. Adding a fuse is a basic precaution, you should size it based on the wire thickness that you are using. You are doing pretty good but beware of knowing 'just enough to be dangerous'. Alternatively once you've finished prototyping the circuit you could hand off the process of packaging it in a way that complies with the various standards to an electrical engineering firm, from a liability perspective that might be a good idea anyway. hth
jrockway 15y ago
I don't think he cares about accuracy of the result; all he needs is a binary on off. So you measure the output of the circuit with the TV off, make a note, measure the output of the circuit with the TV on, make a note, and set the threshold (in software), somewhere in between. Then you know if the TV is off or not. (I would personally have pasted a photocell onto the power LED, if I really needed to know the TV's state, but that's just me. My TV doesn't have a button for "power on", but it responds to a remote code for one.)

lutorm 15y ago

I don't disagree with your advice, but in this case I think it's not so bad. It's a current sensor, there's practically no voltage across those power wires. And assuming he wired it into the ground part of the circuit and not the phase, no voltage to the electronics either. (However, low voltage doesn't mean that an accidentally dropped conductor wouldn't ruin your day, so insulating them would still be a good idea.)

0x12 15y ago
If you look closely you can see a wire running from the mains straight to the sensor board next to the arduino, clearance is on the order of a few mm. All it takes is a little bit of condensation for a nasty accident, the voltage between those two conductors is 110V AC on one side and +5 V with some unknown level of insulation from mains on the other. Making assumptions about which line carries phase and which is neutral can cause very nasty surprises and should never be a factor. You're one broken circuit away from being the shortest path to ground, even in the neutral line (you'll still have the load as a drop resistor but it's not good).